The invention relates to a low-pressure mercury vapour discharge lamp, whose emission mainly lies in three spectral ranges and of which the colour temperature of the emitted light lies in the range of 2000-3000 K., this lamp being provided with a gas-tight discharge envelope transparent to radiation and having a gas filling comprising mercury and a rare gas and with a luminescent layer comprising luminescent materials whose emission mainly lies in the range of 590-630 nm and in the range of 520-565 nm, whilst further means are provided for maintaining a column discharge in the gas filling, the power consumed by the column being at least 500 W/m.sup.2 surface area of the luminescent layer.
Low-pressure mercury vapour discharge lamps, whose emission mainly lies in three spectral ranges, also designated as three-band fluorescent lamps, are known from U.S. Pat. No. 4,176,294 and from Netherlands Patent Specification No. 164,697. These lamps are commonly used in general illumination and have the advantage that they have both a good general colour rendition (colour rendition index R(a, 8) of at least 80) and a high luminous efficacy (up to values of 90 lm/W and higher). This is possible because the emission of these lamps is mainly concentrated in three comparatively narrow spectral bands. For this purpose the lamps contain a red luminescing material whose emission mainly lies in the range of 590-630 nm and a green luminescing material whose emission mainly lies in the range of 520-565 nm. The required emission in the third spectral range, i.e. the range of 430-490 nm, is supplied in many cases by a blue luminescing material. However, the visible radiation emitted by the mercury vapour discharge itself also provides a contribution (i.e. the emission of the 436 nm mercury line) in this spectral range. The lamps emit white light at a given colour temperature, that is to say that the colour point (x,y in the CIE colour coordinate diagram) of the emitted radiation lies on or near the line of the black body radiators. The colour point of fluorescent lamps of low colour temperature is generally chosen to lie preferably slightly above (for example about 0.010 in y coordinate) the line of the black body radiators.
A desired colour temperature of the light emitted by a three-band fluorescent lamp is obtained by a suitable adjustment of relative contributions in the three spectral ranges to the overall emission of the lamp. As the colour temperature of the lamp is lower, the contribution in the blue range of 430-490 nm should be smaller. It follows from the aforementioned Netherlands Patent Specification No. 164,697 that the minimum attainable colour temperature for lamps having an inner diameter of the tubular discharge envelope of about 36 mm is about 2300 K., in which event the lamp need no longer contain a blue luminescing material and all the required radiation in the blue spectral range originates from the blue mercury radiation. In lamps having a smaller inner diameter of the discharge envelope, especially a diameter of about 24 mm, the mercury vapour discharge is found to be more effieient the relative contribution of the blue mercury line being larger. Consequently for these lamps, the minimum attainable colour temperature is found to have a higher value, i.e. about 2500 K.
Three-band fluorescent lamps of the kind mentioned in the opening paragraph are known, for example, from U.S. Pat. Nos. 4,335,330; 4,199,708; and 4,374,340, and are generally of very compact construction and intended to replace incandescent lamps. Due to their compact construction, the luminescent layer in these lamps is heavily loaded, that is to say, the power consumed by the column during operation of the lamp is at least 500 W per m.sup.2 of surface area of the luminescent layer. This is considerably higher than the load of the luminescent lamp layer in the aforementioned lamps having an inner diameter of about 36 and 24 mm, respectively, which load has a value of the order of 300 and 400 W/m.sup.2, respectively. It has been found that in these heavily loaded lamps the relative contribution of the blue mercury radiation is even higher, and that such lamps without a blue luminescing material have a colour temperature of the emitted light of at least about 2700 K. at colour points lying on the line of the black body radiators. As a result of this, and also of their high R(a, 8), these lamps are suitable to replace incandescent lamps.
Hitherto, incandescent lamps have been mainly used for interior illumination. With a view to saving energy, it is often desirable to replace incandescent lamps with fluorescent lamps. A typical value of the colour temperature of an incandescent lamp is 2650 K. However, the use of colour lamps (for example the so-called flame lamps) and dimmers in interior illumination result in colour temperatures down to about 2000 K. A disadvantage of the aforementioned heavily loaded fluorescent lamps is that due to the intense blue mercury radiation they cannot be used in the frequently desired colour temperature range of about 2000 to about 2700 K.
The invention has for its object to obviate the said disadvantage and in general to provide means for shifting the colour point of heavily loaded three-band fluorescent lamps and for reducing the colour temperature, while substantially maintaining the good general colour rendition and the high relative luminous flux.